1. Field of the Invention
This invention relates to a power supply circuit for a computer system or the like, and more particularly to a power limit circuit which is adaptable for varying input voltage, useful in a computer system or similar electronic equipment. In particular, the invention relates to an off-line, power factor corrected (PFC), power supply circuit.
2. Discussion of Prior Art
In the manufacture of computer systems, it is preferable to produce a single model that is operable in a wide variety of settings, rather than tailoring a system to each different setting. A computer unit which is to be operated from AC line power is preferably adapted to accept a wide range of input voltages, 110V or 220V, 50 Hz or 60 Hz, for world-wide utility. Even though the nominal ratings are 110V and 220V, the power supply might allow the computer system to be operable from a low of 90V to a maximum of 265V, without intervention by an operator to make any adjustments via selector switches or the like.
A power supply circuit which allows operation over this wide range of AC line voltages is described by G. A. Hall et al. in "Application Note 33" at pages 226-234 in Applications Handbook, published 1995 by Micro Linear Corporation, 2092 Concourse Drive, San Jose, Calif. 95131. This power supply circuit uses a Micro Linear ML4824 controller as described at pp. 7-143 to 7-152 of Applications Handbook. This controller includes a power factor corrector circuit and a pulse width modulator type voltage control circuit. The power supply circuit described therein accepts an AC input of 80V to 264V and produces the usual computer operating voltages of 3V, 5V and 12V DC.
However, a power supply as described by Hall et al. presents a problem due to the wide range of input currents for differing AC line voltages, assuming a constant power output. For a computer system rated at 750 Watts, for example, the input current would be 8.3 Amp at 90V input, while at 265V input the current would be 2.1 Amp. At the lower AC line voltages, the line current can be precariously close to the trip current of a fuse or circuit breaker that must be in series with the AC power line. The high current condition must be avoided, as the equipment will be thought by a user to be unreliable, when indeed it is the low line voltage condition that is causing the high current. Of course, there will be loss of data and awkward restart chores if the fuse is tripped while operating in a normal running condition. Causing the fuse or circuit breaker to trip is a catastrophic shut down mode that is undesirable unless there is indeed a high current condition.
There are several methods which have previously been used in power supply circuits to limit the output power and prevent permanent damage to the power supply circuits in computer systems of this type. These include fixed current limit, under-voltage lockout, and fixed power limit, or a combination of these.
Fixed current limit and under-voltage lockout protects against fuse clearing, but is ineffective against component damage at high-input voltages. Fixed power limit is effective but does not allow higher output power at high voltages, and thus the power supply cannot be dual rated to take advantage of the ability of the power supply to provide higher output powers at high voltages.
In a typical power supply circuit using these principles, the AC input is monitored by circuits that produce voltage and current values, e.g., VRMS and I.sub.AC inputs are generated. These inputs are fed into a multiplier, producing a representation of power (P=IV), and this result is compared against a reference voltage. When the reference is exceeded, the duty cycle of the power supply is limited and thus the output power is limited. This arrangement, employed in the power supply circuit described by Hall et al. as set forth above, is effective in many situations, but still does not account for a wide range of input voltages, needlessly limiting power in some cases where it is not necessary.